Conventionally, a manufacturing process for a semiconductor device involves a processing step for selectively plasma etching a silicon nitride film such as a SiN film against an underlying silicon oxide film such as a SiO2 film or a SiOC film (carbon-added silicon oxide film). In this regard, there has been known a technique for improving the selectivity of the silicon nitride film against the silicon oxide film (the ratio of an etching rate of the silicon nitride film to an etching rate of the silicon oxide film) by using a gaseous mixture of, e.g., CH3F and O2 as an etching gas (see, for example, Japanese Patent Laid-open Publication No. 2003-229418).
As mentioned above, it has been well known to selectively etch a silicon nitride film against a silicon oxide film by using, e.g., a gaseous mixture of CH3F and O2 as an etching gas. Further, it has been also known that the plasma can be stabilized by adding a rare gas (e.g., Ar) to the etching gas as a dilution gas.
In the above method, an addition of O2 gas increases the selectivity of the silicon nitride film against the silicon oxide film, since the O2 gas contained in the etching gas hardly contributes to the etching of the silicon oxide film. When the selectivity of the silicon nitride film against the silicon oxide film is increased by adding oxygen to the etching gas, however, a micro mask in the form of fine irregularities would be formed on the surface of the silicon nitride film, which is conjectured to be resulting from oxidation, causing the surface of the silicon nitride film to become rough, which, in turn deteriorates characteristics of the semiconductor device.